Emerging Resistance from Antibiotics in Animal Feed

Ours is the first generation to have the means of protecting itself from the most deadly and common infectious diseases like malaria, tuberculosis, HIV, pneumonia and measles. These diseases can be treated with lifesaving drugs, interventions and control strategies that previous generations could only dream about.

No one knows for sure how many antibiotics used in this country are consumed by animals being raised for food — cattle, dairy, cows, pigs, poultry — but one estimate has the number at 20 million pounds annually. Most of the antimicrobials given to food-producing animals are not used to treat sick animals. Instead, these antimicrobials are routinely added to the feed to prevent disease and promote growth. This exposure to antimicrobials has resulted in resistant bacteria.

Figure 1

When we talk about antibiotic resistance issues, what researchers mean is that bacteria can evolve in animals to become resistant to antibiotics. If humans are exposed to these bacteria they may develop infections that could be difficult to treat. This difficulty in finding treatments is thought to be at least partly due to overuse of antibiotics in humans. The practice of giving antimicrobial doses to prevent disease and promote growth in the animal food supply goes back to the 1950s. Surprisingly, these mechanisms to promote a safe food supply have never been fully understood and are still debated by scientists today.

“The increase in bacterial resistance to antimicrobial drugs is a natural phenomenon — an outcome of evolution,” says Linda Grassie, of the FDA’s Center for Veterinary Medicine (CVM). “Any population of organisms — including bacteria — naturally includes variants with unusual traits. In this case, some bacteria have the ability to fend off the action of an antimicrobial, complicating clinician’s efforts to select the appropriate treatment.”

Food animals, she explains, carry organisms that can make people sick, but may not necessarily make the animals sick. While, Salmonella, Campylobacter and E. coli 0157:H7 are commonly found in the intestines of various food animals, these bacteria may not cause illness in animals, but all three bacteria can cause foodborne illness in humans.

“These bacteria develop resistance when exposed to antibiotics given to the animal,” she says. “These bacteria potentially cross contaminate meat at slaughter and then infect humans who eat the meat, particularly if it is undercooked or cross-contaminated after cooking.”

Figure 2

Is Banning Antimicrobials in Animal Feed The Answer?

The impact of such a ban needs to be studied for issues such as incidence of disease in flocks without preventative antimicrobial treatments, the effect on food production and the general food supply, and how to address possible economic losses to farmers.

Several European countries have already banned the use of antimicrobial treatments in their food animals. In 1986, Sweden banned antimicrobial growth promoters. In 1998, the European Union banned four growth promoters — tylosin, spiramycin, bacitractin, virginiamycin — all similar to human drugs. In 1999, Denmark stopped using growth promoters in chicken, beef, and swine; the result was a 60 percent drop in total volume of antimicrobials used in Denmark (see chart page 52). When these EU countries cut their antimicrobial treatments of food animals, resistance rates fell dramatically. The Centers for Disease Control and Prevention (CDC) recently stated that those who make their living in agriculture must do three things to help stem the advance of antimicrobial resistance in

American food animals:

Reduce use in livestock and in other agriculture production ventures since resistance factors can pass through food to consumers.

Support research for non-antimicrobials to enhance growth in food animals.

Follow the new guidelines from the American Veterinary Medical Association (AVMA) for drug use for food animal veterinarians.

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